1. Field of the Invention
The present invention relates generally to sample concentration for analyzing volatile organic compounds in air, water and soils. More particularly, the invention involves a device for removing water vapor from the analyte slug as it is carried from the trap of the sample concentrator to the gas chromatographic column.
2. Related Art
Sample concentrators are used in purge-and-trap, headspace, and thermal desorption gas chromatography ("GC") analysis. Purge-and-trap GC technique has been used for analyzing volatile organics in water since approximately the early 1970's. In 1987 the U.S. Environmental Protection Agency ("EPA") promulgated national primary drinking water regulations for certain volatile organic chemicals ("VOCs"). The EPA also proposed maximum contamination levels for eight volatile organic chemicals. These regulations require the use of the purge-and-trap GC technique. In addition to the eight regulated volatile organic chemicals, the EPA also promulgated monitoring requirements for an additional 52 synthetic volatile organic chemicals.
The EPA has approved certain analytical methods for analyzing these 60 compounds. One of the methods is 502.2, a purge-and-trap capillary-column GC method using a photoionization detector and an electrolytic conductivity detector joined in series. A second method is method 524.2, a purge-and-trap capillary-column GC-MS method.
Purge-and-trap systems for analyzing VOCs in drinking water have been assembled from a variety of equipment typically including a purging device, trap, and desorber. These systems also are referred to as sample concentrators. The purge-and-trap system or sample concentrator interfaces to a GC capillary column, then with a photoionization detector/electrolytic conductivity detector or a mass-spectrometer. These components are interconnected via pneumatic conduits.
Highly volatile organic compounds with low water solubility are extracted (purged) from the sample matrix by bubbling an inert gas (i.e., helium or nitrogen) through a five milliliter aqueous sample. Purged sample components are trapped in a tube containing suitable sorbent materials. When purging is complete, the sorbent tube is heated and backflushed with the inert gas to desorb trapped sample components onto a capillary GC column. The column is temperature programmed to separate the method analytes which are then detected with a photoionization detector (PID) and a halogen specific detector placed in series, or with a mass spectrometer.
Tentative identifications are confirmed by analyzing standards under the same conditions used for samples, and comparing results and GC retention times. Additional confirmatory information can be gained by comparing the relative response from the two detectors. Each identified component is measured by relating the response produced for that compound to the response produced by a compound that is used as an internal standard. For absolute confirmation, the gas chromatography/mass spectrometry (GC/MS) determination according to method 524.1 or method 524.2 may be used.
As stated above, the typical purge and trap system consists of the purging device, trap, and desorber. Systems are commercially available from several sources that meet EPA specifications.
Under EPA specifications, the glass purging device must be designed to accept five to twenty-five ml. samples with a water column at least 5 cm. deep. Gaseous volumes above the sample are kept to a minimum to reduce "dead volume" effects. The purged gas passes through the water column as finely divided bubbles.
The sorbent trap is a tube typically at least 25 cm. long and having an inside diameter of at least 0.105 inches. The trap contains certain sorbent materials which the EPA has specified as 2,6-diphenylene oxide polymer, silica gel, and coconut charcoal. The EPA regulations specify the ratios of the adsorbent material. The desorber must be capable of rapidly heating the trap to 180.degree. C.
The model 4460A sample concentrator manufactured by OI Analytical of College Station, Texas, is an example of a purge and trap, or sample concentrator, device. The model 4460A is a microprocessor controlled device that stores method 502.2 and 524.2 operating conditions as default parameters. Operating conditions may be changed by the user to accommodate other types of purge and trap analysis.
In addition to purge-and-trap methods and analyses, sample concentration gas chromatography is used in headspace analysis of liquids and solids, and in thermal desorption analysis of air tube samples. Headspace and thermal desorption techniques are not only used for environmental analyses, but also for clinical and industrial applications.
During all sample concentration GC analysis, some amount of water is purged from the sample and caught in the trap along with the compounds of interest. This is a problem encountered in the prior art. A typical rate of water transfer is 1 microliter per minute of purge time. Without any water management system, during the 11-minute purge time required by method 502.2, approximately 10 to 11 microliters of water are transferred to the trap. This water transfer as a function of time is represented in FIG. 9. When the trap is heated, the VOCs and the water vapor are desorbed into the GC column. The presence of water vapor in the capillary column has many detrimental effects including shifts of analyte retention times, the quenching of PID response, deterioration of the nickel reaction time in the ELCD, and the suppression of quantification ion response in a mass spectrometer. Along with trapped water, there also may be trapped methanol causing deleterious effects on the separation and detection of VOCs.
FIGS. 1 and 2 are examples of plots showing how water transfer affects ion response of a GC or mass spectrometer ("MS"). The units on the horizontal axis are minutes of GC run time, and the units on the vertical axis are indicative of the abundance of each analyte.
FIG. 1 represents a GC or MS response sensitive to water as well as the analytes of interest. Water transfer during approximately the first six or seven minutes is represented on this plot as a "plateau" during that period. Thus, detection of analytes is difficult during that period.
FIG. 2 represents the same analysis as that of FIG. 1, except the GC or MS is sensitive only to an ion range excluding water. Despite the absence of the plateau from FIG. 1, water transfer during approximately the first six or seven minutes tends to diminish or obscure the peaks of interest during that period. Typically, peaks are lower due to the presence of water vapor. Therefore, the plot of FIG. 2 similarly presents analyte detection problems because of the water transfer problem.
Typically, the methods described above call for a 4 minute desorb period, which is represented on the GC or MS plot as the first 4 minutes of run time. Generally, few if any analytes show up on the GC or MS during approximately the first 4 minutes. However, methane and water begin appearing during that period. After the four minute desorb period, water continues to appear, obscuring the analytes of interest.
Water continues to be transferred from the trap to the GC during the remainder of the run time, or will be limited to approximately the first 6 or 7 minutes of run time, depending on various factors. In general, if water transfer time is reduced, there is greater distortion of results (represented graphically as a "higher" plateau) during that reduced time period. If the water transfer time is extended, the distortion will continue further during the GC run time. Regardless of the length of time during which water transfer occurs, it has a tendency to obscure the analytes of interest.
Devices in the prior art addressing the water transfer problem have involved placing a condenser in line between the trap and GC. However, removing water vapor by condensation has certain disadvantages. One disadvantage is "dead volume," which has deleterious effects on the GC analysis. Due to dead volume, volatile compounds are trapped in the condenser, and GC peaks exhibit "tailing."
Another disadvantage of the condensation approach is that passing of the compound through a cold zone in the condenser interrupts the desired flow of the compound to the GC column. Retention times for analytes are less predictable and repeatable, and a fraction of the analyte may be retarded in the cold zone, reducing the ability to detect and quantify certain compounds. Further, it is desirable to heat the pneumatic lines so that condensation does not occur.
In the OI Analytical model 4460A sample concentrator, the desorption of water vapor onto the GC column is reduced by a water management system that utilizes rapid trap heating at 800.degree. C. per minute coupled with an expansion/condensation chamber that allows only 0.25 microliters per minute of water vapor to desorb onto the GC column. Due to this water management system, over 90% of the trapped water can be rejected. When the chamber is at 35 degrees C., approximately 1.1 microliters of water vapor are desorbed onto the GC column during the 4 minute desorb period. The GC response to water vapor only as a function of time is shown if FIG. 9. The same device at 25 degrees C. delivers approximately 0.93 microliters of water to the GC column. This feature is particularly valuable in detection and measurement of some early eluting compounds whose PID, ELCD or MS responses are normally attenuated by transfer of water vapor to the detector. These systems also reduce water-activated deterioration of the ELCD nickel reaction tube and the subsequent loss of response to certain compounds.
Another example of a condensor-type device is the TEKMAR 2000 Plus purge and trap concentrator, with a moisture control module ("MCM"). The MCM condensate trap cools the VOCs and water that pass through, then preheats the sorbent trap. Upon reaching the preheat temperature, the trap is backflushed with carrier gas sweeping the volatiles and water from the heated trap over to the cooled MCM condensate trap. The water condensed from the gas stream is isolated from the carrier gas flow path, and then is heated to vaporize through a vent. A thermoelectric module (Peltier effect module) is used to cool and heat the MCM in an effort to remove the water vapor from the analyte stream. The Peltier effect module passes electric current through the junction of two thermoelectric materials to cool the MCM, and then to subsequently heat it to remove the water vapor. However, the Peltier module is susceptible to failure after repeated heating and cooling cycles. In addition, the cost of the MCM unit and the electrical requirements are disadvantages of such a water management system.
As another alternative to eliminate water vapor transfer to the GC column, OI Analytical's Anhydrator reduces water transfer to the GC column to less than 0.004 microliters per minute. The Anhydrator consists of Nafion tubing available from Perma-Ture Corporation. The Anydrator has disadvantages and problems including the irreversible loss of polar analytes such as acetone and methanol, which are removed along with the water.